I found this to be the easiest part of the assembly process. The extruder is the mechanical device that pushes the filament all the way down to the hot end. The extruder proper, is a simple stepper motor, attached to the top plate of the 3D printer and pushes thef filament down a Bowden tube.
Here’s a picture of the extruder before I attached it to the top plate.
Here’s a picture of the extruder and the Bowden tube connected to the hot end. For display purposes, I have reversed the arms of the hot end.